ABSTRACT

Type 51 R bodies are produced by all bacterial endosymbionts (Caedibacter taeniospiralis) of Paramecium tetraurelia that confer the hump-killer trait upon their hosts. Type 51 R-body synthesis by C. taeniospiralis is required for expression of the hump-killer trait. The genetic determinants for type 51 R-body synthesis by C. taeniospiralis 47 have been cloned and expressed in Escherichia coli. In this communication we describe three species of polypeptides required for R-body synthesis and the organization of their genetic determinants. Each polypeptide species is controlled by a separate gene that is expressed as an independent transcriptional unit possessing regulatory signals that are recognized by E. coli. Two polypeptide species of 10 and 18 kilodaltons are required for R-body synthesis but apparently are not structural subunits. The third polypeptide species (13 kilodaltons) is the major structural subunit. R-body assembly involves polymerization reactions that result in high-molecular-mass polypeptide complexes, primarily composed of the 13-kilodalton polypeptide species, that appear to be the result of covalent cross-linking between structural subunits. The results presented here have been suggested to apply to the assembly and structure of all type 51 R bodies, but not necessarily to other R-body types.